Hamburger patties are frequently manufactured at a central location, using high-speed high-volume patty molding machines, and subsequently distributed to restaurants, grocery stores, and other retail outlets. Patties of flaked or shredded meat, fish, and vegetable foods may also be handled in this manner. The term "food product", as used throughout this specification and in the appended claims, refers to any of the various foods identified above and to others having similar properties; these food products are not free-flowing, but rather are quite viscous and resistant to flow, and are only moderately compressible.
In many high-volume patty molding machines, the food product is fed from a supply hopper into a food pump by a positive feed mechanism that forces the food product into the pump intake. A positive feed of this kind is used to overcome the viscous, flow-resistant properties of the food product. The food pump forces the food product, under pressure, into a mold cavity in a mold plate that is moved cyclically between a fill position and a discharge position.
One problem encountered in these food patty molding machines is excessive "churning" or "working" of the food product by the food pump and its associated feed mechanism. Churning may be caused by the positive feeding action used to force the food product into the pump intake, with circulatory flow resulting because the feed mechanism cannot be precisely matched to the intake capacity of the pump. Another source of churning results from the tendency of the food pump to force food product back into the supply hopper, particularly when an open-intake plunger pump is employed. Churning tends to grind the food product into smaller particles than desired and may also cause separation of fat from other tissue in ground meat or separation of water from a fish or vegetable food product. A vacuum intake pump of the kind described and claimed in Sandberg et al U.S. Pat. No. 4,054,967 materially reduces the churning problem, as compared with forced-intake pumps, but may not eliminate it entirely.
In many of these machines a mold plate change may require adjustment of the pump cycle for the most efficient operation. Thus, if a thin mold plate with only two small cavities is replaced by a thick mold plate with three cavities of nearly the same diameter a much larger quantity of food product must be pumped in each cycle and the pump feed mechanism needs adjustment to assure adequate filling of the mold cavities. To a lesser extent, a change in the compressibility of the food product might also require adjustment. These adjustments have sometimes been difficult and time-consuming, particularly in coordinating separate adjustments for the food pump and its feed mechanism.
Hydraulically driven plunger-type food pumps are preferred, in food patty molding machines, because they permit effective operation at relatively low but constant pressures, a substantial advantage in avoiding undesirable bulges or "lips" on the food patties due to the action of entrained air. On the other hand, the efficiency of the plunger pump sometimes is lower than desired, requiring a relatively large pump for limited volumes of food product. This is particularly true with a vacuum-intake plunger pump if the pump allows any substantial quantity of the food product to be driven back into the supply hopper from the pumping chamber. Thus, although the quality of the patties may justify the expense, a vacuum intake pump may entail greater capital expenditure than is desirable (oversize motor, etc.). Furthermore, with a plunger-type food pump, the desirability of two operating pressures, a low pressure for the pump stroke of the plunger and a high pressure for rapid plunger retraction, tends to require capital expenditures of an undesirably high level.